An essential octamer motif in the mb-1 (Igalpha) promoter

The regulatory interplay between Oct-1 isoforms contributes to hematopoiesis and the isoforms imbalance correlates with a malignant transformation of B cells

Oct-1(POU2F1) is a DNA-binding transcription regulator and its level being highly increased in many human cancers. Oct-1 is present in the human cells as a family of functionally different isoforms which are transcribed from alternative promoters. Here, we have demonstrated that expression patterns of Oct-1 isoforms change during differentiation of hematopoetic progenitor cells (CD34+) (HPCs) to the B (CD19+) and T (CD3+) cells. While Oct-1L is expressed at a high level in the CD34+ HPCs, its expression level drops dramatically during the T-cell differentiation, although remains nearly the same in B-cells. We have described the novel human Oct-1R isoform which is conserved in mammals and is B cell-specific. Oct-1R was found in B cells, but not in HPCs. Oct-1R is transcribed from the same promoter as Oct-1L, another lymphocyte-specific isoform. Overexpression of Oct-1R and Oct-1L in the Namalwa cells leads to the repression of many genes involved in B-lymphocyte differentiation and signal transduction. Thus these isoforms may regulate the particular stages of development of normal B cells and maintain their proper differentiation status. However the extremely high level of Oct-1L isoform observed in the B-lymphoblast tumor cell lines indicated that the excess of Oct-L seem likely to considerably decrease the differentiation ability of these cells. Oct-1 may serve as a therapeutic target for many tumors, but it should be noted that in a tumor the content of a certain isoform Oct-1, rather than the total Oct-1 protein, can be increased.

The mouse B cell-specific mb-1 gene encodes an immunoreceptor tyrosine-based activation motif (ITAM) protein that may be evolutionarily conserved in diverse species by purifying selection

The B-lymphocyte accessory molecule Ig-alpha (Ig-α) is encoded by the mouse B cell-specific gene (mb-1), and along with the Ig-beta (Ig-β) molecule and a membrane bound immunoglobulin (mIg) makes up the B-cell receptor (BCR). Ig-α and Ig-β form a heterodimer structure that upon antigen binding and receptor clustering primarily initiates and controls BCR intracellular signaling via a phosphorylation cascade, ultimately triggering an effector response. The signaling capacity of Ig-α is contained within its immunoreceptor tyrosine-based activation motif (ITAM), which is also a key component for intracellular signaling initiation in other immune cell-specific receptors. Although numerous studies have been devoted to the mb-1 gene product, Ig-α, and its signaling mechanism, an evolutionary analysis of the mb-1 gene has been lacking until now. In this study, mb-1 coding sequences from 19 species were compared using Bayesian inference. Analysis revealed a gene phylogeny consistent with an expected species divergence pattern, clustering species from the primate order separate from lower mammals and other species. In addition, an overall comparison of non-synonymous and synonymous nucleotide mutational changes suggests that the mb-1 gene has undergone purifying selection throughout its evolution.

Differential lineage-specific regulation of murine CD45 transcription by Oct-1 and PU.1

Although it has been established that CD45 expression is regulated at the transcriptional level, neither the regulatory elements that are responsible for its unique expression pattern nor the relevance of its three distinct transcriptional start sites (P1a, P1b, and P2) has been fully characterized. We studied the contribution of the three start sites to CD45 mRNA production in haematopoietic cell lines and primary haematopoietic cells. In myeloid and lymphoid cells and cell lines most CD45 transcripts originate from P1b with the exception of the thymoma-derived T cell line EL4, in which approximately 90% of CD45 transcripts originate from P1a. The degree of contribution of P1a is highest in lymphoid cells and increases in T cells following mitogen stimulation. In vitro evaluation of sequence upstream of the start sites shows that the P2 start site is sufficient for CD45 expression in lymphoid but not in myeloid cells, confirms the presence of a PU.1-binding site essential for myeloid expression of CD45, and reveals an Octamer-binding site that interacts with both Oct-1 and Oct-2 and activates CD45 transcription in lymphoid and myeloid cells. These findings are the first evidence that Octamer-binding factors are involved in the control of CD45 expression.

Regulatory mechanisms that determine the development and function of plasma cells

Plasma cells are terminally differentiated final effectors of the humoral immune response. Plasma cells that result from antigen activation of B-1 and marginal zone B cells provide the first, rapid response to antigen. Plasma cells that develop after a germinal center reaction provide higher-affinity antibody and often survive many months in the bone marrow. Transcription factors Bcl-6 and Pax5, which are required for germinal center B cells, block plasmacytic differentiation and repress Blimp-1 and XBP-1, respectively. When Bcl-6-dependent repression of Blimp-1 is relieved, Blimp-1 ensures that plasmacytic development is irreversible by repressing BCL-6 and PAX5. In plasma cells, Blimp-1, XBP-1, IRF4, and other regulators cause cessation of cell cycle, decrease signaling from the B cell receptor and communication with T cells, inhibit isotype switching and somatic hypermutation, downregulate CXCR5, and induce copious immunoglobulin synthesis and secretion. Thus, commitment to plasmacytic differentiation involves inhibition of activities associated with earlier B cell developmental stages as well as expression of the plasma cell phenotype.

Impaired hematopoiesis in mice lacking the transcription factor Sp3

As the zinc-finger transcription factor specificity protein 3 (Sp3) has been implicated in the regulation of many hematopoietic-specific genes, we analyzed the role of Sp3 in hematopoiesis. At embryonic day 18.5 (E18.5), Sp3-/- mice exhibit a partial arrest of T-cell development in the thymus and B-cell numbers are reduced in liver and spleen. However, pre-B-cell proliferation and differentiation into immunoglobulin M-positive (IgM+) B cells in vitro are not affected. At E14.5 and E16.5, Sp3-/- mice exhibit a significant delay in the appearance of definitive erythrocytes in the blood, paralleled by a defect in the progression of differentiation of definitive erythroid cells in vitro. Perinatal death of the null mutants precludes the analysis of adult hematopoiesis in Sp3-/- mice. We therefore investigated the ability of E12.5 Sp3-/- liver cells to contribute to the hematopoietic compartment in an in vivo transplantation assay. Sp3-/- cells were able to repopulate the B- and T-lymphoid compartment, albeit with reduced efficiency. In contrast, Sp3-/- cells showed no significant engraftment in the erythroid and myeloid lineages. Thus, the absence of Sp3 results in cell-autonomous hematopoietic defects, affecting in particular the erythroid and myeloid cell lineages.

The regulation of the Oct-1 gene transcription is mediated by two promoters

The ubiquitous transcription factor Oct-1 is a member of the POU domain family of regulatory proteins. Target genes controlled by Oct-1 include housekeeping genes, e.g. the genes encoding histon H2B or snRNAs, as well as tissue-specific genes, e.g. the genes encoding the light and heavy chains of immunoglobulines, some interleukins, and others. Oct-1 pre-mRNA may be spliced in several ways, resulting in production of several protein isoforms that may differ functionally. The 5'-end of the Oct-1 gene contains two exons-exon 1U and exon 1L that alternatively present in Oct-1 mRNA. We studied regulation of transcription of the Oct-1 gene using reporter gene assays of promoter-luciferase gene-constructs. It was shown that transcription of the Oct-1 gene is regulated by two promoters located upstream of the exon 1U and upstream of the exon 1L. The promoter located upstream of the exon 1U contains G/C-rich sequences and multiple Sp1 sites, while the promoter located upstream of the exon 1L contains A/T-rich motifs and autoregulation-related cis-elements: two octamer sites ATGCAAAT, two octamer related sites and multiple TAAT-core sites. Exons 1U and 1L in the human OTF-1 locus encoding the Oct-1 gene are located at the distance of 108 kbp. In the murine locus otf-1 the distance between exons 1U and 1L is 67 kbp. We suggest that the two promoters can differ functionally.

Human Oct-1L isoform has tissue-specific expression pattern similar to Oct-2

POU homeodomain proteins are important regulators of ubiquitous as well as tissue-specific transcription. Ubiquitously expressed Oct-1 and tissue-specific Oct-2 proteins are members of the POU family and contain very similar DNA-binding POU domains. While Oct-1 is ubiquitous, Oct-2 is predominantly expressed in B cells, in activated T cells and in nervous system. Oct-1 is involved in regulation of some houskeeping genes-histone H2B, snRNAs as well as in tissue-specific regulation of immunoglobuline gene transcription and of some other genes. Here we report that novel alternatively spliced product of the human Oct-1 gene encode Oct-1L isoform with tissue-specific expression pattern, similar to Oct-2. Oct-1L differ from ubiquitously expressed Oct-1A in 5'-terminal exon (exon 1L). Analysis of nucleotide sequences from Human Genome Data Bank has located exon 1L about 108 kbp downstream ubiquitously expressed exon 1U. Amino terminus of Oct-1L show extensive similarity to amino terminus of Oct-2. We suppose, that Oct-1L may has a specific role in gene expression in lymphoid tissues and brain.

Early B-cell factor, E2A, and Pax-5 cooperate to activate the early B cell-specific mb-1 promoter

Previous studies have suggested that the early-B-cell-specific mb-1(Igalpha) promoter is regulated by EBF and Pax-5. Here, we used in vivo footprinting assays to detect occupation of binding sites in endogenous mb-1 promoters at various stages of B-cell differentiation. In addition to EBF and Pax-5 binding sites, we detected occupancy of a consensus binding site for E2A proteins (E box) in pre-B cells. EBF and E box sites are crucial for promoter function in transfected pre-B cells, and EBF and E2A proteins synergistically activated the promoter in transfected HeLa cells. Other data suggest that EBF and E box sites are less important for promoter function at later stages of differentiation, whereas binding sites for Pax-5 (and its Ets ternary complex partners) are required for promoter function in all mb-1-expressing cells. Using DNA microarrays, we found that expression of endogenous mb-1 transcripts correlates most closely with EBF expression and negatively with Id1, an inhibitor of E2A protein function, further linking regulation of the mb-1 gene with EBF and E2A. Together, our studies demonstrate the complexity of factors regulating tissue-specific transcription and support the concept that EBF, E2A, and Pax-5 cooperate to activate target genes in early B-cell development.

Silencing of B cell receptor signals in human naive B cells

To identify changes in the regulation of B cell receptor (BCR) signals during the development of human B cells, we generated genome-wide gene expression profiles using the serial analysis of gene expression (SAGE) technique for CD34(+) hematopoietic stem cells (HSCs), pre-B cells, naive, germinal center (GC), and memory B cells. Comparing these SAGE profiles, genes encoding positive regulators of BCR signaling were expressed at consistently lower levels in naive B cells than in all other B cell subsets. Conversely, a large group of inhibitory signaling molecules, mostly belonging to the immunoglobulin superfamily (IgSF), were specifically or predominantly expressed in naive B cells. The quantitative differences observed by SAGE were corroborated by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and flow cytometry. In a functional assay, we show that down-regulation of inhibitory IgSF receptors and increased responsiveness to BCR stimulation in memory as compared with naive B cells at least partly results from interleukin (IL)-4 receptor signaling. Conversely, activation or impairment of the inhibitory IgSF receptor LIRB1 affected BCR-dependent Ca(2+) mobilization only in naive but not memory B cells. Thus, LIRB1 and IL-4 may represent components of two nonoverlapping gene expression programs in naive and memory B cells, respectively: in naive B cells, a large group of inhibitory IgSF receptors can elevate the BCR signaling threshold to prevent these cells from premature activation and clonal expansion before GC-dependent affinity maturation. In memory B cells, facilitated responsiveness upon reencounter of the immunizing antigen may result from amplification of BCR signals at virtually all levels of signal transduction.

Identification of E2A target genes in B lymphocyte development by using a gene tagging-based chromatin immunoprecipitation system

The transcription factors encoded by the E2A gene are known to be essential for B lymphocyte development, and ectopic expression or gene inactivation studies have revealed several potential lineage-specific E2A target genes. However, it remains unknown whether these target genes are directly regulated by E2A at the transcriptional level. We therefore generated mice carrying an affinity-tagged E2A knock-in allele to provide a system for the direct elucidation of E2A target genes based on E2A binding to target regulatory regions. Abelson-transformed pre-B cell lines derived from these mice were used in chromatin immunoprecipitation experiments to identify regulatory sequences bound by E2A in the context of an early B lymphocyte environment. Significant E2A binding was detected at the promoters and enhancers of several essential B-lineage genes, including the Igkappa intronic and 3' enhancers, lambda5 and VpreB surrogate light chain promoters, the EBF locus promoter region, and the mb-1 (Igalpha) promoter. Low levels of E2A binding were observed at several other lymphoid-restricted regulatory regions including the Ig heavy chain (IgH) intronic enhancer, the IgH 3' enhancers hs3b/hs4, the RAG-2 enhancer, and the 5' regions of the B29 and TdT loci. An E2A target gene, the predicted butyrophilin-like gene NG9 (BTL-II), was also identified by using a chromatin immunoprecipitation-based cloning strategy. In summary, our studies have provided evidence that E2A is directly involved in the transcriptional regulation of a number of early B-lineage genes.

Bob1 (OCA-B/OBF-1) differential transactivation of the B cell-specific B29 (Ig beta) and mb-1 (Ig alpha) promoters

The B29 (Igbeta) and mb-1 (Igalpha) gene products are B cell-specific essential components of the B cell receptor that are coexpressed at all stages of B cell differentiation, with the exception of plasma cells, which lack mb-1 expression. Transcription of both genes is governed by a similar cassette of interactive transcription factor-binding elements, including octamer motifs, in TATA-less promoters. In this study, we show the B cell-specific B29 gene promoter is transactivated in B and non-B cells by cotransfection with the B cell-specific octamer cofactor gene, Bob1 (OCA-B/OBF-1). The expression of Bob1 is also sufficient to override the silencing effects of the B29 silencer. This indicates that Bob1 plays a critical role in B cell-specific B29 promoter expression. In contrast, coexpression of Bob1 had no effect on mb-1 promoter activity. Bob1 transactivation only occurs with select octamer sequences that have an adenosine at position 5 (ATGCAAAT). The B29 promoter conforms to this consensus octamer motif, while the mb-1 promoter octamer motif does not. Octamer motif swapping between B29 and mb-1 promoters renders B29 unresponsive to Bob1 transactivation and makes mb-1 competent for Bob1 transactivation, thereby indicating that the B29 octamer motif is solely responsible for Bob1 interaction. Additionally, the mb-1 construct containing the B29 octamer motif is expressed in a plasmacytoma cell line, while the wild-type mb-1 promoter is not. Bob1 transactivation of B29 and the lack of this transactivation of mb-1 account for the differential expression of B29 and mb-1 in terminally differentiated plasma cells.